Customizable plugin, that nicely displays related posts thumbnails under the post.
If your theme supports WordPress Posts Thumbnail feature and post has a specified Post Thumbnail as seen on Edit Post page, when this image will be used as a post thumbnail in related posts. The size of the image is determined by plugin setting 'Post-thumbnails name'.
If your theme does not support Post Thumbnail feature or post has no assigned thumbnail, when the first image on the post will be used as a thumbnail.
There is no direct support of external images in the current version of the plugin. You can use custom field option. Or you can use http://wordpress.org/extend/plugins/generate-post-thumbnails/ or similar plugin to save external images as post thumbnails.
If there are no images of the specified size in the post, or file does not exist on the server the default image will be used as a post thumbnail. You can upload your image on server and specify its URL in plugin settings.